The present invention generally relates to a so-called surface-acoustic-wave device having a plurality of interdigital electrodes, and in particular to the structure and fabrication process for eliminating the occurrence of sparking discharges between the fingers of the electrodes during the process of device fabrication.
Recently, the demand for increased operational speed of information processing apparatuses and communication apparatuses has caused the shift of the frequency of for the carriers or signals to higher frequency regions. In correspondence to such a shift of the frequency, filters capable of operating in such high frequency regions are required. For this purpose, the surface-acoustic-wave (abbreviated hereinafter as SAW) devices are used.
In view of the expected developments in the future, Particularly in the field of automobile telephones and portable telephones, efforts are being made to develop SAW devices having a sharp attenuation in the frequency region outside the pass-band while maintain a uniform band-pass characteristic. By using the SAW devices in place of the conventional dielectric filters, the size of the filters can be reduced to about 1/30th of the size of conventional filters and the size of the telephone can be reduced accordingly.
A typical SAW device, such as the SAW filter, uses a piezoelectric substrate having large electromechanical coupling coefficients and small temperature coefficient of frequency. For example, a single crystal of LiTaO.sub.3 is used widely. The crystal of LiTaO.sub.3 is cut in a predetermined orientation, and interdigital electrodes are provided on the substrate as the input and output electrodes.
FIG. 1 shows the geometrical parameters characterizing a typical interdigital electrode.
Referring to FIG. 1, the electrode comprises a first part EL1 and a second part EL2 each respectively having a number of fingers f.sub.1 -f.sub.n and g.sub.1 -g.sub.n, wherein each finger has a width W and is separated from adjacent fingers by a separation S. Designating the wavelength of the surface acoustic wave as .lambda., the width W and the separation S are generally set to satisfy the relation W=S=.lambda./4. Thereby, the pitch defined in FIG. 1 as P is set to P=.lambda./2. Further, each finger in the electrode EL1 and each finger in the electrode EL2 are provided to form a uniform overlap as shown in FIG. 1. Such an electrode is called the uniform overlap electrode.
When forming a SAW filter having a central band pass frequency of 835 MHz, for example, the pitch P is set to 2.45 .mu.m while the width W and the separation S are set to 1.23 .mu.m in correspondence to the velocity of 4090 m/sec of the surface acoustic wave in the X-direction. It should be noted that the foregoing velocity provides the wavelength .lambda. of 4.9 .mu.m for the surface acoustic wave of 835 MHz. Generally, a pair of such electrodes EL1 and EL2 are provided. In the Particular applications of SAW devices such as automobile telephones or portable telephones, on the other hand, devices having a small insertion loss, a wide Pass-band and a large suppression for the frequency components outside the pass band, are required. For example, an insertion loss of 3-5 dB or less, a pass band of 25 MHz or more a side lobe suppression of 24-25 dB or more may be required for the SAW filter having the central frequency of 835 MHz.
In order to satisfy these various requirements, the applicants of the present invention have proposed devices and previously disclosed in the United States, European, Korean and Canadian patent applications entitled "SURFACE-ACOUSTIC-WAVE FILTER HAVING A PLURALITY OF ELECTRODES," based upon the Japanese patent applications 2-69121 and 2-86236, and which are incorporated herein by reference.
FIG. 2 shows a wafer 1 on which a number of SAW devices 2 are formed. Typically, the SAW devices 2 are arranged in the rows and columns with dicing lines 3 formed between adjacent SAW devices 2. When the formation of the SAW devices 2 is completed, the wafer 1 is subjected to a dicing Process wherein the wafer 1 is cut by a diamond saw along the dicing lines 3. Thereby, the SAW devices 2 are separated from each other.
FIG. 3 shows a typical conventional example of the SAW device 2 before the dicing process is carried out.
Referring to FIG. 3, the device 2 is surrounded by the dicing line 3, and a number of input electrodes 21 and a number of output electrodes 22 are formed alternately on the surface of the substrate 1 to form a row of electrodes. Each of the input electrodes 21 comprises a first part, corresponding to the part EL1 of FIG. 1, connected to a common input bonding pad 28 and a second part, corresponding to the part EL2 of FIG. 1, connected to a ground pad 27. The ground pad 27 is provided in correspondence to each interdigital electrode 21. Similarly, each output electrode 22 comprises a first part connected to a common output bonding pad 28' and a second part connected a ground pad 27'. Further, a pair of reflectors 25 of the open strip type are formed at both sides of the row of electrodes 21 and 22. The dicing line 3 is merely a hypothetical line for dicing the wafer into the individual devices.
Meanwhile, during the fabrication of the conventional SAW devices, there arises a problem in that, associated with various heating processes employed during the fabrication, the surface of the Piezoelectric substrate is charged due to the pyroelectricity. It should be noted that, in the structure of FIG. 3, the input electrodes 21 and the output electrodes 22 are isolated from each other. Further, in each input and output electrodes, the second part connected to the ground pad 27 or 27' is isolated from each other. In such a structure, the electric charges induced by the pyroelectric effect are accumulated and such accumulation of electric charges induces a sparking discharge between adjacent fingers of the interdigital electrodes. As the separation between the fingers is in the order of several microns in correspondence to the wavelength of the surface acoustic waves, a small amount of such electric charges is sufficient to cause such sparking.
FIG. 4 shows an electrode wherein such sparking has occurred. It should be noted that such sparking discharge damages the finger of the interdigital electrodes and hence the SAW device. In FIG. 4, the fingers 50a and 50b represent the fingers that have experienced the sparking discharge. Thereby, the yield of the device is inevitably deteriorated.